Science of Synthesis

Poli, R.; Smith, K. M., Science of Synthesis, (2003) 2, 312.

The examples outlined in Section ‌2.6.4.5.2‌ are stoichiometric in chromium(II) chlo­ride and generally employ a large excess of this reagent. As shown in ‌Scheme 30‌, reaction of the organic halide with two equivalents of chromium(II) halide yields the desired organochromium species ‌72‌ and one equivalent of chromium(III) halide. The nucleophile then adds to the al­de­hyde with formation of a chromium alk­ox­ide species. The high stability of the oxy­gen—chromium(III) bond serves as the thermodynamic sink; the alcohol product is recovered by hy­dro­lysis in the stoichiometric process. The use of halo­si­lane, however, forces an exchange by virtue of the higher oxo­philicity of silicon, producing an additional equivalent of chromium(III) halide. At this point the reaction can be made catalytic in chromium by simply using a reagent capable of reducing chromium(III) to chromium(II), e.g. metallic manganese. This modification does not compromise the scope, practicability, efficiency, and chemo- and diastereoselectivity of the C—C bond formation.[‌130‌,‌132‌,‌133‌] In addition, it reduces the consumption of the rather high-cost and toxic chromium reagent and paves the way for potential applications in enantioselective syntheses using chromium catalysts with chiral ancillary ligands.[‌134‌]

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References

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• 8.Meeeee-Meee, (8888) M 88e, 888.